This invention relates generally to lithium-sulfur batteries, and in particular to battery electrolytes having additives of oxidizing agents.
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries. The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for batteries having high specific energies. In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in rechargeable nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries are also suitable for use in lower power applications such as cameras or portable recording devices.
At this time, the most common secondary batteries are probably the lead-acid batteries used in automobiles. These batteries have the advantage of being capable of operating for many charge cycles without significant loss of performance. However, such batteries have a low energy to weight ratio. Similar limitations are found in most other systems, such as Ni-Cd and nickel metal hydride systems.
Among the factors leading to the successful development of high specific energy batteries, is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery has been sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium provides also greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable primary and rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). The patents disclose a sulfur-based positive electrode for a battery cell that has low equivalent weight and high cell voltage and consequently a high specific energy (greater than about 120 Wh/kg). The disclosed positive electrode addresses deficiencies in the prior art to provide a high capacity sulfur-based positive composite electrode suitable for use with metal (such as lithium) negative electrodes. These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 mAh/g (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 milliamp-hours per gram (mAh/g) of sulfur.
Nevertheless, the challenge of providing improved batteries, and especially batteries having increased cycle life and long shelf-life, remains. In particular, the shelf-life of lithium-sulfur batteries is limited by the degradation of the lithium electrode surface during cell storage and the formation of thick and resistive surface passivating film comprising Li2S. The passivating film may significantly increase the lithium electrode polarization at the early stages of the cell discharge.
To compensate for this loss of active anode material, extra lithium may be provided for the lithium electrode increasing the cost and weight of the battery. The use of additional metals also increases the burden of disposing of the battery as additional toxic materials must be processed. Mossy lithium formed during cell cycling can also present a fire hazard by creating fine particles of lithium metal that can ignite on contact with air.
Accordingly, methods for the prevention of capacity loss in battery cells with sulfur-containing cathodes and the prevention of degradation of the surface of a lithium electrode in such cells would be desirable.
The present invention provides oxidizer-treated lithium electrodes, battery cells containing such oxidizer-treated lithium electrodes, battery cell electrolytes containing oxidizing additives, and methods of treating lithium electrodes with oxidizing agents and battery cells containing such oxidizer-treated lithium electrodes. Battery cells containing an SO2 oxidizing agent as an electrolyte additive in accordance with the present invention exhibit higher discharge capacities after cell storage over cells not containing SO2. Pre-treating the lithium electrode with SO2 gas prior to battery assembly prevented cell polarization. Moreover, the SO2 treatment does not negatively impact sulfur utilization and improves the lithium""s electrochemical function as the negative electrode in the battery cell.
One aspect of the invention provides a battery cell electrolyte. The battery cell electrolyte may be characterized as including: a) a main solvent of an electrolyte solvent mixture, having the chemical formula R1(CH2CH2O)nR2, where n ranges between 1 and 10, R1 and R2 are different or identical groups selected from the group consisting of alkyl, alkoxy, substituted alkyl, and substituted alkoxy groups and b) an oxidizing agent additive comprising no more than about 49% by weight of the electrolyte solvent mixture. The oxidizing agent additive may be at least one of sulfur dioxide, nitrous oxide, carbon dioxide, a halogen, an interhalogen, an oxychloride and a sulfur monochloride where the halogen is selected from the group consisting of Cl2, Br2 and I2 In specific embodiments, the oxychloride may be selected from the group consisting of SO2Cl2 and SOCL2 and the interhalogen may be selected from the group consisting of iodine monochloride (ICl), iodine trichloride (ICl3) and iodine monobromide I2Br2 Typically, the oxidizing agent additive has a stronger oxidizing ability than elemental S.
In preferred embodiments, the electrolyte may include a dioxolane co-solvent where the dioxolane co-solvent comprises less than about 20% by weight of the electrolyte solvent mixture and a second co-solvent having a donor number of at least about 13. The main solvent may be from the glyme family, in particular 1,2-dimethoxyethane (DME). The electrolyte may include an electrolyte salt where the electrolyte may be in a liquid state, a gel state or a solid state.
Another aspect of the present invention provides a battery cell. The battery cell may be characterized as including: a) a negative lithium electrode b) a positive electrode comprising an electrochemically active material and c) an electrolyte including a: i) a main solvent of an electrolyte solvent mixture, having the chemical formula R1(CH2CH2O)nR2, where n ranges between 1 and 10, R1 and R2 are different or identical groups selected from the group consisting of alkyl, alkoxy, substituted alkyl, and substituted alkoxy groups and ii) an oxidizing agent additive. The oxidizing agent additive may be at least one of sulfur dioxide, nitrous oxide, carbon dioxide, halogens, interhalogens, oxychlorides and sulfur monochlorides. The electrochemically active material may comprise sulfur in the form of at least one of elemental sulfur, a metal sulfide, a metal polysulfide, an organosulfur material, and combinations thereof, wherein said metal is selected from the group consisting of alkali metals, alkaline earth metals, and mixtures of alkali and alkaline earth metals.
In specific embodiments, the battery cell electrolyte may include dioxolane as a co-solvent, comprising no more than 20% by weight of the electrolyte solvent mixture and a high donor number co-solvent having a donor number of at least about 13. In addition, the battery cell electrolyte may include an electrolyte salt. The electrolyte of the battery cell may be in a liquid state, a gel state, or a solid state.
Another aspect of the present invention provides a method of making a protected lithium electrode battery cell. The method may be characterized as including: a) treating a lithium material with an oxidizing agent to form a negative electrode having a protective film, b) forming a positive electrode comprising an electrochemically active material and c) combining said negative and positive electrodes with an electrolyte following the treating of said lithium material where the oxidizing agent is at least one of sulfur dioxide, nitrous oxide, carbon dioxide, halogens, interhalogens, oxychlorides and sulfur monochlorides.
Another aspect of the present invention provides a method of making a protected lithium electrode battery cell. The method may be characterized as including: a) forming a negative electrode comprising a lithium material, b) forming a positive electrode comprising an electrochemically active material and c) combining said negative and positive electrodes with an electrolyte containing an oxidizing agent additive wherein the oxidizing agent additive reacts with the lithium material of the negative electrode to form a protective film on the negative electrode""s surface. In specific embodiments, the negative electrode may be a glassy coated lithium electrode where a crack in the glassy coated lithium electrode may be penetrated by the oxidizing agent additive and the crack may be filled with a reaction product between the oxidizing agent additive and the lithium material of the glassy coated lithium electrode.
These and other features of the invention will further described and exemplified in the drawings and detailed description below.